drm/amd/display: Refactor headless to use atomic commit. (v2)

Headless mode set needs to be synchronized against outstanding nonblocking
commits. This achieved by building atomic state and commiting it.

v2: rebase on 4.11

Signed-off-by: Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>